YES GW DID it first.
The Umbra pattern boltgun was first used in the later years of the Great Crusade and was produced in big quantities.
The variant known as Umbra Ferrox was mass-produced by ChapterTrivia I Techmarines to ensure that enough weapons and ammunition were available during the ongoing Horus Heresy. The difference to the "normal" Umbra pattern is the replacement of the original sickle clip by a box magazine, allowing for a greater ammunition capacity. Also incorporated are a long-range gunsight with additional Target Reciprocator Systems which mesh with the Power Armour's Autosenses.
Note: contrary to the statement that the "Umbra-Ferrox variant" is different only in the ammunition clips used, other sources show "Umbra-Ferrox patterns" with sickle clips.